In the manufacture of certain articles, e.g., ceramic parts, higher densities and improved properties may be achieved when a low density preform of the article is densified by a hot isostatic pressing (HIP) sintering process. For some materials, e.g., ceramic materials and ceramic composites, it is necessary to hermetically encapsulate the preform in order to prevent the gas used during HIP processing from infiltrating the preformed body and preventing consolidation. Glass is often applied on or around the preformed body to form the encapsulation. The glass is plastic at sintering temperatures and acts as a pressure transfer medium for the gas to consolidate the material.
Unfortunately, the glass encapsulation can itself cause problems. The glass may interact with, e.g., bond to or react with, the material so as to cause damage to the article. Also, a difference between the expansion coefficients of glass and the material may cause cracking or breaking of thin sections, and surface damage to large bodies, during the cooling portion of the HIP process.
It is consequently necessary to use a release coating between the article, e.g., ceramic part, and the encapsulant which does not react with article surface yet prevents the encapsulant from interacting with the part. The use of such a release coating greatly simplifies the encapsulation technique for complex shaped parts because it allows the use of encapsulants whose thickness approaches that of the part itself, an issue when thin-walled articles are needed.
It has been proposed in U.S. Pat. No. 4,717,535 to use boron nitride (BN) as a release agent. In that patent, the release coating is formed of two or more layers, the inner layer consisting substantially of BN, and the outer layer consisting of a mixture of BN and an additive material which increases the viscosity of the encapsulation glass during processing. This second layer reduces the ability of the glass encapsulation to penetrate through to the part. Pure boron nitride powder coatings are soft with very low strength. Parts coated with it cannot be handled using ordinary processing procedures. Handleability is somewhat improved by the addition of a second layer described in the patent, but the prior art process still leaves a fragile coating on the ceramic preforms. A single layer release coating having good strength and handleability would clearly be advantageous and would eliminate the complexity of applying a two-layer system.
Besides the aforementioned U.S. Pat. No. 4,717,535, the prior art in the field includes: U.S. Pat. No. 4,112,143 which describes a silicon nitride encapsulating technique but does not address the problem of removal and/or reaction of the encapsulant with the ceramic; U.S. Pat. No. 4,264,546 which relates to a psuedo-isostatic hot press process in which boron nitride is used as a pressure transfer medium during the process; U.S. Pat. No. 4,562,817 which applied boron nitride to alumina spacers to prevent reaction between the alumina and the silicon nitride during processing; U.S. Pat. No. 4,568,516 which describes a process for forming a boron nitride layer during processing through a reaction of boron oxide with the nitride of the ceramic material being processed; U.S. Pat. No. 4,612,146 which uses an amorphous carbon coating as the gas-impervious layer for isostatic processing; and U.S. Pat. No. 4,692,288 which discloses a process in which carbon powder is used as the pressure transfer medium. In addition, U.S. Pat. No. 4,094,799 discloses various solid materials, including graphite and boron nitride as additives for lubricating oil compositions. This patent is not concerned with release coatings or methods in forming release coatings, as described herein.